A keyboard is the part of a computer system that enables the user to control certain aspects of the computer. In general, a keyboard includes a standard set of printable characters, usually laid out in the QWERTY pattern, function keys F1, F2 . . . , and a number of nontext keys, such as Control, Alt, and Shift. The nontext keys are used to change the meaning of another key. The standard set of printable characters are hereinafter called "standard key". Each of the standard keys functions itself, namely, each means something itself. On the other hand, the nontext keys are always used together with some of the standard keys, because the nontext keys do not mean anything themselves. In the keyboard, a number of key switches corresponding to the keys are arranged in matrix on the intersections of drive lines and receive lines. The keyboard is provided with a micro-controller which performs predetermined processing in response to key press and key release operations.
In a wireless keyboard, a micro-controller scans the key matrix to detect which keys are pressed, and transmits key data corresponding to the pressed keys via infrared or radio waves. For interlock function of keys, N-key-roll-over technique is generally used. In this technique, pressed keys are scanned in order, and are transmitted to an upper device, such as a personal computer. For instance, keys "a," "b" and "c" are pressed successively, the corresponding key data are transmitted in order. In the same manner, for the nontext keys, such as SHIFT and CTRL, the corresponding key data are transmitted to the upper device. For example, when SHIFT(L) key and "a" key are pressed at the same time, the key data of SHIFT(L) are first transmitted and the key data of "a" follows. Ii response to the two pieces of key data, the upper device recognize character "A" (=SHIFT(L)+"a").
In accordance with N-key-roll-over technique, the key codes are transmitted in the order of key scan, regardless of the function of the keys. With a wired keyboard, no transmission error occurs.
With the wireless keyboard, however, transmission errors may occur due to outside noise, interference, directivity of infrared, and the like. For instance, in the case where the key data of "a" is not transmitted to the upper device, although a combination of SHIFT and "a" have been pressed at the same time, the upper device can not recognize anything. That is because the nontext key SHIFT does not mean anything itself. The upper device, however, can finally recognize the demanded key data "A" when key press of "a" is repeated. On the other hand, if the key data of SHFT are not transmitted to the upper device, the upper device recognize the key data of "a" without any doubt. That is the problem.